The present invention relates to a photomask which is used to manufacture a semiconductor device and the like, and more particularly to a photomask which has been subjected to a processing of shifting a phase of exposure light beams and a pattern forming method employing the same.
Along with an increase of the integration scale for semiconductor devices, sizes of patterns for forming constituent elements of the devices become fine, and sizes equal to or smaller than the critical resolution of a projection aligner are required. As a method of fulfilling such a request, in JP-B-62-50811 published on Oct. 27, 1987, and corresponding to JP-A-57-62052 (laid open on Apr. 14, 1982) for example, a photomask is employed in which a transparent film for shifting a phase of exposure light beams is provided on a transparent portion on one of the opposite sides sandwiching an opaque portion, and thus the resolution of a pattern is exceptionally improved.
In the above-mentioned prior art, a phase shifter needs to be arranged in one of the transparent portions adjacent to each other, and for the arrangement of the phase shifter in the complicated element pattern, high trial and error is necessarily required. Thus, there is required considerable labor. In addition, since the number of processes of manufacturing a photomask is doubled as compared with the prior art, the reduction in yield and the increase in cost become problems.
Those problems can be settled by employing a semitransparent phase shifting mask in which a semitransparent portion and a transparent portion are provided, and some of the light beams passed through the semitransparent portion are phase-inverted with respect to light beams having passed through the transparent portion. With respect to this point, the description will hereinbelow be given with reference to the accompanying drawings.
FIG. 1A is a cross sectional view showing a structure of an example of a semitransparent phase shifting mask. In the figure, reference numeral 1 designates a transparent substrate, and reference numeral 2 designates a semitransparent film. A thickness of the semitransparent film 2 is adjusted such that the light beams having passed through the transparent portion 3 are phase-inverted with respect to the light beams having passed through a semitransparent portion 4. The semitransparent film 2 has a transmittance such that a light beam having passed through the transparent substrate 1 and the semitransparent film 2 has an intensity high enough to cause interference with a light beam having passed through the transparent substrate 1. The transparent film used in this specification means a film having the above-mentioned transmittance.
The light intensity distribution of the projected light beams on a wafer becomes, as shown in FIG. 1B, a sharp light intensity distribution. The reason such a sharp light intensity distribution is obtained is that since the light beams having passed through the transparent portion are phase-inverted with respect to the light beams having passed through the semitransparent portion, the former and the latter cancel each other in a boundary portion of the pattern so that the light intensity becomes approximately zero. In addition, since the intensity of the light beams having passed through the semitransparent portion is adjusted to the be equal to or lower than the sensitivity of a photoresist, the intensity of the light beams having passed through the semitransparent portion is not an obstacle to the formation of the pattern. That is, in this method, since the phase inversion effect between the pattern to be transferred and the semitransparent portion therearound is utilized, there is no need to take, as in the normal phase shifting mask, the arrangement of the phase shifter into consideration. In addition, in the prior art phase shift mask, the two lithography processes are required for the formation of the mask. However, in this method, one lithography process has only to be performed. Thus, it is possible to form the mask very simply.
In this method, the light beams, which have an intensity that is equal to or lower than the sensitivity of a photoresist to which the pattern of the mask is to be transferred are made to pass through the semitransparent film so that the light beams which have passed through the semitransparent film are phase-inverted with respect to the light beams which have passed through the transparent portion, and thus, the contrast of the pattern is improved. As a result, it is possible to improve the resolution of an aligner for transferring the mask pattern. The basic principle of the semitransparent phase shifting mask is described in D. C. Flanders et al.: "Spatial Period Division-A New Technique for Exposing Submicrometer-Linewidth Periodic and Quasiperiodic Patterns" J. Vac. Sci. Technol., 16(6), Nov./Dec. pp 1949 to 1952 (1979), U.S. Pat. Nos. 4,360,586 and 4,890,309 and JP-A-4-136854 (laid open on May 11, 1992).
In the lithography process in which the above-mentioned semitransparent phase shifting mask is employed, in the normal exposed area, good pattern formation can be performed. However, it has been made clear by the investigations made by the present inventors that since in the actual exposure of the wafer, the mask pattern is repeatedly transferred by step-and-repeat exposure, the light beams which have leaked from the semitransparent area, which is located outside the periphery of the actual pattern element corresponding to an active region of a substrate, leak out to the adjacent exposed area, and thus this is an obstacle to good pattern formation.
It is therefore an object of the present invention to provide a photomask by which a good pattern can be obtained even in the case of an exposure, in which a mask pattern is repeatedly transferred by step-and-repeat exposure, and a pattern forming method employing the same.
According to one aspect of the present invention, the above-mentioned object can be attained by effectively making a light-shielding or opaque area of a semitransparent phase shift mask which is located outside the periphery of a pattern element formation area of the semitransparent phase shifting mask.
The light shielding portion in the semitransparent phase shifting mask is formed by processing a semitransparent film to obtain a pattern having a width equal to or lower than the resolution. The reason for adopting such a method is that if a light shielding film is newly formed as the light shielding portion, this will result in an increase of the number of processes of forming the mask. Incidentally, by optimizing the area ratio of the semitransparent portion to the transparent portion, it is possible to further effectively form the light shielding portion.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings.